Alfalfa Harvest Strategy Effect on Lygus Bug (Hemiptera: Miridae) and Insect Predator Population Density: Implications for Use as Trap Crop in Cotton

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Abstract
Lygus bug, Lygus hesperus Knight, and beneficial insect populations ( Orius sp., Geocoris spp., and Nabis spp.) were quantified in alfalfa strips, which were interplanted within cotton fields. The propensity of alfalfa strips to retain lygus bugs, thereby keeping them out of cotton, and the utility of alfalfa strips as a reservoir for beneficial insects were evaluated from May to August in 1990 and 1991. The following three alfalfa cutting strategies were examined: (1) uncut alfalfa, (2) a 28-d strip-cut treatment, and (3) a 35-d strip-cut treatment. Insect populations were quantified in both the newer alfalfa growth and the older alfalfa growth of the strip-cut treatments. Lygus bug densities were significantly higher in the uncut treatment than in the other treatments, peaking at 520 per 1.9 m 2 in 1990 and 350 per 1.9 m 2 in 1991. Lygus bug levels in the strip-cut treatments were low in the newer alfalfa growth (averaging 41.8 per 1.9 m 2 ) and only slightly higher in the older alfalfa growth of the 28-d strip-cut treatment (averaging 65.3 per 1.9 m 2 ). Populations were relatively constant within these three treatments throughout the season. However, in the older growth of the 35-d cutting regime, Lygus bug populations averaged 103 per 1.9 m 2 . Densities of beneficial insects were also higher in the uncut than the strip-cut treatments, but the higher predator densities apparently did not offset the high Lygus bug population increase. The uncut treatment also lacked Lygus bug egg and nymphal mortality from the alfalfa cutting as compared with the strip-cut treatments. The less frequent cutting, in the 35-d compared with the 28-d treatment, probably also allowed for a buildup of the Lygus bug population. Lygus bug instar distribution, sex, and predator species were also examined in these treatments. It appears that to optimize Lygus bug management, while still producing high densities of predacious insects, strip-cut alfalfa should be used and cut on a 28-d schedule.

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